Receptor binding
A. In the simplest circumstance, agonist occupancy of the receptor obeys the law of mass action, and the relationship between agonist concentration (linear scale) and response is reflected by a rectangular hyperbolic relationship. B. A plot of response versus log [agonist] reveals a sigmoidal relationship between occupancy and response, such that, in the absence of negative or positive cooperativity, 10% to 90% response occurs over approximately a 100-fold range of agonist concentration, "centered" about the EC50 for agonist. C. Agonists vary in terms of potency and efficacy. The EC50 value represents the concentration of agonist that elicits a half-maximal response. Drug L is more potent than drugs M N; drugs L and N are more efficacious than drug M, a partial agonist. D. Because occupancy often is not directly related to response and signal amplification occurs between receptor occupancy, effector activation, and ultimate response, dose-response curves often fall to the left of receptor-occupancy profiles.